1. Field of the Invention
The present invention relates to telecommunications. The invention more particularly relates to wireless telecommunications apparatus, systems and methods which implement data transmission via a plurality of telecommunication channels such as radio channels with variable parameters. More specifically, the invention relates to wireless systems with multicarrier transmission, although it is not limited thereto.
2. State of the Art
In wireless data transmission systems, a signal is subjected to several frequency conversions with respective shifting of its carrier frequency and initial phase. In mobile systems, the carrier frequency is additionally subjected to the Doppler effect. In addition, the signal phase at the receiving point depends on the time interval of radio signal propagation in the communication channel, and this time interval is changed because of both the change of the signal propagation path and the change of properties and parameters of the propagation media. In wireless multipath channels, the change of any single interference component (its amplitude or/and phase) causes the change of the received signal phase as a whole. As a result, the initial signal phase has a constant component and a varying, typically slowly changing component. Usually, in wireless systems, the constant component is compensated in the receiver during the preamble by estimating frequency offset and frequency equalizer adjustment utilizing a special pilot signal.
Optimal signal processing in data transmission systems and wireless telecommunication systems is based on certain a priori information about received signals and channel characteristics. This information includes symbol time interval, carrier initial phase, signal attenuation, signal-to-noise ratio and other service parameters, which are extracted from the received signal by means of special functions such as clock synchronization, carrier recovery, signal equalization, channel estimation, etc. In channels with variable characteristics, such as multipath wireless channels, the above-mentioned service parameters change over time, and their estimation, in order to remain current, requires special adaptive or tracking procedures.
Typically in wireless systems, service parameter estimation and tracking are based on utilization of special pilot signals. Two types of pilot signals are usually used: preamble pilots transmitted during a preamble before data transmission, and accompanying pilots transmitted during the whole communication session in parallel with data transmission. As a rule, these two types of pilots have not only different parameters but also provide different functions.
The preamble pilot consists of few symbols and takes a comparatively small part of the communication session. It is used for automatic gain control (AGC), clock synchronization, initial frequency offset correction, preliminary carrier phase adjustment, as well as for channel parameters estimation. For example, in a WLAN system according to the IEEE802.11a standard, the preamble pilot contains two training sequences: a short training sequence, and a long training sequence. The short training sequence consists of ten short OFDM symbols with duration 0.8 μs, and the long training sequence consists of two long OFDM symbols with duration 3.2 μs. Each short OFDM symbol is a sum of twelve phase-modulated carriers with numbers: 2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50. Each long OFDM symbol is a sum of all fifty-two phase modulated carriers. The short and long training sequences are separated by a guard interval with a duration of 1.6 μs. The total duration of the preamble pilot signal (training signal) is 16 μs, which is 80% of a whole service signal, transmitted before data, but it is a very small part of the communication session as a whole.
The IEEE standard specifies that the short training sequence should be “used for AGC convergence, diversity selection, timing acquisition, and coarse frequency acquisition in the receiver”, and the long training sequence should be “used for channel estimation and fine frequency acquisition in the receiver” (Section 17.3.2.1). So, the preamble pilot, as a rule, does not considerably decrease the average data rate of the system (system capacity), and this type of pilot signal is not the focus of this invention.
In contrast to the preamble pilot signal, the accompanying pilot signals are usually transmitted during the whole communication session in parallel with data transmission. The accompanying pilot signals are typically used for adaptive equalization, for frequency offset tracking, and for current adjustment of carrier phases to provide improved coherent signal processing. For example, in the WLAN system according to the IEEE802.11a standard, the accompanying pilot signal consists of four pseudo-randomly modulated carriers. The standard specifies: “In each OFDM symbol, four of the carriers are dedicated to pilot signals in order to make coherent detection robust against frequency offset and phase noise. These pilot signals shall be put in carriers −21, −7, 7, 21. The pilots shall be BPSK modulated by a pseudo binary sequence to prevent the generation of spectral lines” (Section 17.3.5.8). So, in the OFDM WLAN system forty-eight carriers are used for data transmission and four carriers are dedicated to pilot signals; i.e., about 8% of the system capacity, as well as transmitter power, is used for pilot signal transmission.
Approximately the same portion of the system capacity is wasted in the fixed wireless broadband systems according to the IEEE802.16 standard (Section 8.3.5.3.4), in which one constant pilot carrier is used per twelve data carriers.
It should be noted that a decreasing real data rate is not the only disadvantage of pilot utilization. When using frequency spaced (i.e., frequency-separated) pilots for phase adjustment of the carrier signals, the accuracy of the phase adjustment is not sufficient for perfect coherent processing, especially in multipath wireless channels. As a matter of fact, the phases of the frequency spaced carriers are not 100% correlated. Therefore, even if the estimation of a pilot phase is perfect, the estimation of an adjacent carrier phase may be not correct. Taking into account this fundamental disadvantage of pilot systems, the authors of the IEEE802.16 standard have proposed to use variable location pilot carriers in addition to the constant location pilot carriers. Variable pilots shift their location each symbol with a cyclic appearance. This technique allows a receiver to improve phase tracking accuracy, but it leads to complicated synchronization and additional capacity loss.
It should also be noted that existing approaches to pilotless phase tracking system design are based on carrier recovery techniques. See, J. Proakis, “Digital Communications”, 4th edition, McGraw-Hill, 2001, Section 6.2. Carrier recovery techniques provide individual phase tracking for each carrier. They provide simple and efficient solution for single carrier systems with small-size constellations, but they are practically unacceptable for multicarrier systems with multipoint QAM constellations.